The present invention relates to thermostat having a bimetal temperature sensing member and related manufacturing method, and more particularly to such a thermostat having design features providing self calibration to define the predetermined threshold temperature of the thermostat.
The provision of an accurately calibrated thermostat is a prime concern to many product manufacturers today. Home appliances, such as coffee makers, tea kettles, corn poppers, hair dryers and the like must be carefully temperature controlled to provide an even temperature and to prevent overheating. Likewise, other consumer products from computers to automobiles utilize a large number of thermostats for the same or similar purposes.
There are two basic types of thermostats that are widely commercialized for these and related purposes; namely, a creep thermostat characterized by components that move continuously, and a snap thermostat where the components move suddenly or discontinuously. The snap thermostat category is divided into two basic types, one with a blade type bimetal member and the other with a disc type bimetal member. Because of the competitiveness of thermostat manufacturers, the design of thermostats and the related manufacturing processes are today highly sophisticated in a technical sense. Of all of the requirements with regard to the technical development of thermostats, the overriding consideration is quality and safety of the final product. Of course, each manufacturer is also concerned with holding the cost to manufacture down commensurate with the degree of quality and safety that is required.
In the past, some designs of thermostats have fallen short of these goals. This is particularly true with regard to thermostats that have substituted a thin walled glass reinforced plastic body for ceramic construction. In these devices, the thin wall is prone to cracking and can result in failure or an electrical shock hazard. Other thermostats are of a design that require a manufacturing process that tends to result in damage or loosening to the delicate electrical contacts and terminals. Other thermostats utilizing caps that are spin rolled or staked to the thermostat body can cause cracking of the structure, and in extreme cases this can cause generation of sufficient internal dust in the thermostat to cause a coating of the contacts and interruption of the proper thermostat operation. In some cases, there have been ill-advised attempts to maintain the cap as thin as possible which simply compounds these difficulties. Also, some extreme spin rolling and staking manufacturing techniques in the past tend to cause distortion of critical components within the thermostat. This in turn inherently causes a change in the critical internal dimensions, including the spacing between the contacts, further interfering with the proper operation.
Further with regard to the manufacturing processes of the thermostat, some have required a high degree of precision of manufacturing to provide an acceptable level of calibration. An example of this is the accepted tolerance for an actuating shaft between the bimetal member and the resilient blade supporting the movable contact. One prior thermostat requires a close tolerance of plus or minus two ten thousandths inch (.+-.0.0002 inch). Because of the close tolerance situation in these prior thermostats, manufacturers have resorted to such time consuming additional steps of selection and matching of components in order to obtain the proper calibration.
A typical example of a prior art snap thermostat is illustrated in the Schmitt U.S. Pat. No. 3,636,622. As can be seen by analysis of the thermostat illustrated in the '622 patent, some of the shortcomings of the prior art are inherent in its design. Furthermore, the manufacturing process is complicated in that the parts are combined so as to use simple friction for retention during assembly. Because the use of simple friction in this manner requires highly accurate and expensive components, the calibration of the thermostat in this manner is undesirable. Furthermore, the design of the thermostat and the manufacturing process is such that compensation must be introduced for varying ambient temperature and other parameters. Final retention of the parts is attempted by the use of adhesive, which is not only inherently unreliable, but costly. It is also inherently messy in its application.
In view of these combined design and manufacturing shortcomings, the present invention is characterized by a thermostat comprising novel components combined to optimize the integrity and accuracy of the finished thermostat. A part of the invention is also directed to the manufacturing method that is simplified from the methods of the past, and at the same time results in improvements to the calibration of the thermostat. The method is characterized by a self calibrating step during the assembly and without the need for compensation.